1. Field of the Invention
The present invention relates to a heat-sensitive recording material and particularly relates to a heat-sensitive recording material having improved storage stability of an uncolored portion and a developed color image. Further, the invention relates to metal salts of salicylic acid derivatives which are useful as an electron accepting compound in the heat-sensitive recording material and a process for preparing said salicylic acid derivatives and metal salts of the same.
2. Related Art of the Invention
Heat-sensitive recording materials which utilize color forming reaction of electron donating compounds with electron accepting compounds(developer) have conventionally been well known, for example, in Japanese Patent Publication Sho 43-4160 and 45-14039.
Heat-sensitive recording materials are relatively inexpensive and advantageous in that recording equipment is compact and free from maintenance, and hence have widely been used in the field of a facsimile, recorder and printer.
Recently, use field of heat-sensitive recording materials has been further extended and diversified to uses under severe environment, for example, labels and pre-paid cards.
However, conventionally known heat-sensitive recording materials which employ 2,2-bis(4'-hydroxyphenyl)propane(bisphenol A) or benzyl 4-hydroxybenzoate as an electron accepting compound have such a disadvantage that remarkable soiling of an uncolored portion or fading of a developed color image occurs in the severe environment, for example, in contact with oils, solvents, fats and writing utensils such as a fluorescent pen or in moist environment.
In order to improve these disadvantages, heat-sensitive recording materials which contain salicylic acid derivatives having substituted amino groups or metal salts thereof as electron accepting compounds have been proposed (Japanese Laid-Open Patent Sho 63-95979).
However, the heat-sensitive recording material which uses 4-phenylacetylaminosalicylic acid or a metal salt such as zinc salt of the same which described in the patent as an electron accepting compound is still poor in the storage stability of a developed color image against moist heat, has low sensitivity for color development and cannot conform to practical high-speed recording. The heat-sensitive recording material obtained as described in the patent by using 4-benzoylaminosalicylic acid and metal salts(for example, zinc salt) thereof as an electron accepting compound has also poor storage stability, for example, moist heat resistance of a developed color image and a disadvantage of low developing sensitivity. Further, the heat-sensitive recording material obtained by using a similar compound 4-n-octylcarbonylaminosalicylic acid or its metal salts such as zinc salt as an electron accepting compound has also the same disadvantages as above. On the basis of such circumstances, it has been strongly desired to develop a heat-sensitive recording material having good developing sensitivity and further improved storage stability of the uncolored portion and developed color image.
Relatively many salicylic acid derivatives having an amino group have been known to be used for heat-sensitive recording materials as disclosed above.
The salicylic acid derivative which is disclosed in the invention has a carbamate group as a substituent and is represented by the formula (1). Some of the salicylic acid derivative represented by the formula (1) have already been prepared, for example, in German Patent 2749518, Bull. de Socie. Chim. France, 1189 (1955), and J. Pharm. Sci., 52, 927 (1963). The sodium salt of the salicylic acid derivative is the only metal salt which has been disclosed and no disclosure has been found at all on divalent, trivalent and tetravalent metal salts. As to the application to the heat-sensitive recording material, no disclosure has been found on the salicylic acid derivative having the carbamate group as a substituent or a metal salt of the derivative.
The above German Patent 2749518 has disclosed a preparation process of 4-benzyloxycarbonylaminosalicylic acid and 5-benzyloxycarbonylaminosalicylic acid by individually reacting 4-aminosalicylic acid and 5-aminosalicylic acid with benzylchloroformate in dry pyridine. However, the yield is very low. For example, the yield of 5-benzyloxycarbonylaminosalicylic acid in the process is about 25%.
In the preparation process described in Bull. de Socie. Chim. France, 1189 (1955), 4-alkyloxycarbonylaminosalicylic acid is obtained by reacting an aqueous sodium 4-aminosalicylate solution with alkylchloroformate. However, the yield is not satisfactory in the process. For example, the yield of 4-n-pentyloxycarbonylaminosalicylic acid is about 70%.
In the preparation process described in J. Pharm. Sci., 52, 927 (1963), 4-aminosalicylic acid and 5-aminosalicylic acid is reacted with phenylchloroformate in an aqueous sodium hydrogen carbonate solution to obtain 4-phenyloxycarbonylaminosalicylic acid and 5-phenyloxycarbonylaminosalicylic acid, respectively, in the relatively high yield.
However, the present inventors have found that, when 4-aminosalicylic acid or 5-aminosalicylic acid is reacted in an aqueous sodium hydrogen carbonate solution with various chloroformate compounds, particularly alkylchloroformate compounds having more than 5 carbon atoms, the yield of desired 4-carbamate substituted salicylic acid derivative or 5-carbamate substituted salicylic acid derivative is very low (50% or less).
Consequently, it has been strongly desired to develop a simple and broadly applicable process for preparing a carbamate substituted salicylic acid derivative in high purity.